High density is one of the key desired attributes for materials used in applications such as kinetic energy penetrators, aircraft and helicopter balance blades, and radiation shields. Kinetic energy penetrators are used, for example, in military applications for piercing and penetrating heavy armor found on tanks and armored personnel carriers. In kinetic energy penetrators, generally, the higher the density of the material, the greater the desired penetration. For aircraft and helicopter balance blades, the aim is to concentrate the maximum possible weight in the smallest possible space. And for radiation shields, higher density results in higher absorption of X-rays and gamma radiation. Thus, economical high density materials in bulk shapes, have many important applications. However, most of the high density materials (with densities greater than 16 or 17 g/cc) like gold, rhenium, tungsten, osmium, iridium, uranium, etc., are either very expensive or extremely difficult to process.
For example, there are only two known types of economically viable and operationally successful kinetic energy penetrating materials: depleted uranium (DU) and tungsten heavy alloys. Both types of materials possess the high density, strength and ductility combination which is required for ballistic penetration. However, existing forms of these alloys suffer from significant limitations. At present, DU is known to be a more effective penetrator material than an equivalent-density tungsten heavy alloy. Mounting political pressure and increasing environmental issues have created conditions where DU penetrators, in spite of their superior effective penetration characteristics, are being discarded in favor of tungsten heavy alloys.
The superior performance of DU is attributable to its ability to fail through the formation of adiabatic shear. This results in constant "self-sharpening" during penetration of the DU penetrator that prevents a "mushrooming" effect that can otherwise occur. Mushrooming during ballistic penetration produces a resisting pressure on the projectile over a large area that causes increasing deceleration force and detrimentally affects penetration efficiency. State-of-the-art tungsten heavy alloys based on W-Ni-Fe do not self-sharpen, but mushroom instead and thereby lose penetration efficiency. If a tungsten heavy alloy could be produced which failed during penetration in a localized adiabatic shear, then the penetration performance would be similar to that of the DU kinetic energy penetrators.
Yet another limitation associated with tungsten heavy alloys is inherent in the tungsten itself. Tungsten which has a density of 19.3 g/cc is brittle and requires extremely high processing temperatures (around 2,000.degree. C.) to obtain full density. Tungsten heavy alloys that have been developed avoid this problem, by essentially "gluing" in a lower density ductile alloy matrix. The matrix alloy consists of nickel alloyed with either iron, copper, cobalt, and some amount of tungsten which is taken into solution. This ductile matrix serves a two-fold purpose. First, it imparts ductility to the brittle tungsten, and second, it lowers the process temperature of the tungsten heavy alloy.
The typical liquid-phase sintering temperature for known tungsten heavy alloys varies from 1450.degree. to 1650.degree. C. While this sintering temperature is significantly lower than that of pure tungsten, it is still too high for general powder metallurgical production furnaces. For example, U.S. Pat. No. 4,938,799 to Nicolas filed on Oct. 5, 1988 and entitled "Heavy Tungsten-Nickel-Iron Alloys with Very High Mechanical Characteristics and Process for Production of Said Alloys" discloses an alloy with a specific gravity of between 15.6 and 18. These alloys include a tungsten .alpha.-phase in the shape of butterfly wings with dislocation cells of dimensions between 0.1 and 1 .mu.m and a Ni-Fe bonding gamma-phase having a mean free path of less than 15 .mu.m and a Ni/Fe ratio greater than or equal 2.0. The alloy of that invention requires a sintering temperature of between 1490.degree. and 1650.degree. C. for from 2 to 5 hours and further requires strengthening by way of special thermal chemical treatments. These temperatures and processes are usual beyond the capability of general powder metallurgical production furnaces and thus may add significantly to the production costs of the alloy or at least limit the accessibility of the alloy for many applications.
Thus there is a need for an inexpensive high-density alloy that can be used for a variety of purposes.
There is a need for an inexpensive high-density alloy that does not suffer from the environmental issues relating to DU, but can be used as successfully for kinetic energy penetrators.
There is a further need for a tungsten heavy alloy that does not suffer from the known mushrooming effects typical of existing tungsten heavy alloys. In particular, there is a need for a tungsten heavy alloy that during ballistic penetration fails through the formation of adiabatic shear.
Further, there is a need for a tungsten heavy alloy that can be produced by conventional ferrous powder metal part manufacturers and other conventional non-ferrous powder metal part manufacturers without the need for reconfiguring their furnaces.